1. Field of the Invention
The present invention relates to an ignition coil for internal combustion engines and, more particularly, to an ignition coil holding a core, a primary winding and a secondary winding in a casing.
2. Description of the Prior Art
An ignition coil for internal combustion engines generally has primary and secondary windings wound concentrically with each other. The starting end of winding of a wire forming the primary winding is electrically connected to a positive side of a battery, and the terminal end of winding of the wire is also electrically connected to a negative side of the battery through a control circuit. The starting end of a wire used as the secondary winding is electrically connected to spark plugs of the internal combustion engine, while the terminal end thereof is electrically connected to the starting end of the primary winding.
Each end of the wires forming the primary and secondary windings is electrically connected to one of plurality of terminals, respectively, protrusively provided on a bobbin for winding the primary winding, a bobbin for winding the secondary winding, and/or on a casing. Each of these terminals is connected to the battery, the control circuit, and the spark plugs.
In the Japanese Utility Model Registration Application Laid-Open Gazette No. 2-13720 issued January, 1990, has been disclosed an ignition coil of such a construction that a coil assembly with primary and secondary windings coaxially wound on the outer periphery of a primary cylindrical bobbin having a square cross section is inserted in a casing produced of an insulating synthetic resin, and then after filling of an insulating synthetic resin in the casing to set the primary and secondary winding with the resin thus filled, a couple of cores formed in the U shape are installed with their legs on one side inserted into a hollow portion of the primary bobbin through each of openings formed on the casing, oppositely from both ends of the bobbin, and with the free ends of their other legs secured by welding on the exterior of the casing. This ignition coil, however, apts to rust readily because the core forming a closed magnetic circuit is partly exposed outside of the casing.
Also disclosed in the Japanese Utility Model Registration Application Laid-Open Gazette No. 2-35429 issued March, 1990, is a process which includes the steps of inserting, into a coil casing, a coil assembly in which a secondary bobbin wound with a secondary winding is retained coaxially on the outer periphery of a primary bobbin wound with a primary winding, protruding the free ends of the legs of a U-shaped core having two legs of the same length out of the exterior of the coil casing, with one of the legs exposed out of the coil casing and with the other legs of the core inserted in the hollow portion of the primary core, fixing the core of small volume to one of the two legs of the core, and then molding the casing portion to be connected to the coil casing and at the same time covering the core portion exposed out of the coil casing. According to the above-noted disclosure, it is difficult to mold the casing portion and therefore is difficult, owing to its construction, to support the terminals on the casing portion.